Abstract
At a low COD:TN ratio (≤5) in influent, maintaining a longer HRT (≥9 h) and longer SRT (≥30 d) are suggested to improve higher N removal efficiency in case of operation at low DO (Dissolved oxygen) level (0.9 ± 0.2 mg-O2/L). However, in case of operation at high DO level (4.0 ± 0.5 mg-O2/L), short HRT (1 h) and typical SRT (17 d) make it possible to achieve nitrogen removal. On the other hand, at a high COD:TN ratio (≥8.4), a typical HRT (9–15 h), SRT (12–19 d), and DO level (1.3–2.6 mg-O2/L) would be applied. Microbial distribution analysis showed an abundance of AOA (Ammonia-oxidizing archaea) under conditions of low DO (≤0.9 mg-O2/L). Nitrosomonas sp. are mostly found in the all investigated water resource recovery facilities (WRRFs). Nitrosospira sp. are only found under operating conditions of longer SRT for WRRFs with a low COD:TN ratio. In comparison between abundances of Nitrobacter sp. and Nitrospira sp., abundances of Nitrobacter sp. are proportional to low DO concentration rather than abundance of Nitrospira sp. A predominance of nosZ-type denitrifiers were found at low DO level. Abundance of denitrifiers by using nirS genes showed an over-abundance of denitrifiers by using nirK genes at low and high COD:TN ratios.
Highlights
Organic matter and inorganic nutrients are the main contaminants to be treated in municipal wastewaters
Organic matter and inorganic nutrients from municipal wastewaters need to be removed before being discharged to our environment
The effects of the chemical oxygen demand (COD):total nitrogen (TN) ratio on the abundance of nosZ-type denitrifiers were consistent with the results reported by Yuan et al [35] who reported that the abundance of nosZ-type denitrifiers was two orders of magnitude higher at an influent
Summary
Organic matter and inorganic nutrients (nitrogen, N and phosphorus, P) are the main contaminants to be treated in municipal wastewaters. Discharge of inorganic nutrients into the environment is responsible for eutrophication or algal blooms and toxic effects to aquatic life. For this reason, organic matter and inorganic nutrients from municipal wastewaters need to be removed before being discharged to our environment. Pre-anoxic systems, which include Modified Ludzack–Ettinger (MLE), step feed, and sequencing batch reactor (SBR), are popular. These systems consist of an anoxic tank (first zone) followed by an aerobic tank (second zone) and are designed for N removal. An anaerobic system prior to anoxic and aerobic systems is designed and operated biologically, in which there is an abundance of microorganisms responsible for both N and P removals. With alkalinity provided for the nitrification step (aerobic zone) and produced denitrification step (anoxic zone), N can be removed efficiently and a good settling sludge can be produced
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